1. Field of the Invention
The present general inventive concept relates to a disk drive system, and more particularly to a read circuit of a disk drive system and a method of signal processing of the read circuit.
2. Description of the Related Art
A hard disk drive (HDD) is used as a standard data storage device in a computer system. The HDD has been widely used as a data storage device because manufacturing costs are low, storage capacity is large, power dissipation is low, and the speed of data transmission is fast.
The disk drive system includes at least one rotating magnetic platter and many devices for reading and writing data to and from the rotating magnetic platters. A read/write head is located over the rotating magnetic platters. A disk drive circuit is coupled to the read/write head, and controls a position of the read/write head and generates and senses an electromagnetic field that represents data to be read and to be written. The disk drive circuit receives data from a host device, such as a personal computer (PC), and transforms the data into magnetic codes. The read/write head writes the magnetic codes on the rotating magnetic platters. Further, when a host device requests data from the disk drive system, the disk drive circuit searches for locations of data to be read, senses magnetic codes that represent the data, and transforms the magnetic codes into binary digital information. The disk drive circuit may include an algorithm for error detection and error correction.
As the density and rotation speed of the magnetic platters are increased, a data processing technology referred to as Partial Response Maximum Likelihood (PRML) has been developed in order to solve the problems related to peak detection. PRML adopts an algorithm that is implemented in the disk drive system in order to interpret magnetic signals that are sensed by the read/write head.
The disk drive system based on PRML reads waveforms of analog signals generated by magnetic flux reversals that are stored on the disk platters. The disk drive system based on PRML does not search for peak values to represent the magnetic flux reversals. The disk drive system, however, digitally samples a waveform of an analog signal (partial response), and determines bit patterns for the waveform (maximum likelihood). The PRML technology is insensitive to noise that may be included in the detected magnetic signals. Therefore, even a platter of low quality may be used and manufacturing yield may be increased, and manufacturing costs may be decreased.
In general, a read circuit of a disk drive system may include a sequence detector for recovering a data sequence from equalized data to generate recovered data, and a signal-dependent post-processor for correcting a signal-dependent error included in the recovered data.
In the disk drive system, there may exist unwanted noise components such as additive white Gaussian noise (AWGN), media noise, signal-dependent noise and magnetoresistive (MR) asymmetric noise. Among the noise components, the signal-dependent noise, which is non-linear noise, is not fully eliminated.
Nowadays, research is conducted on eliminating the signal-dependent noise. A technology for reducing non-linear noise included in an output data of a Viterbi detector using an error event filter is disclosed in U.S. Pat. No. 5,949,831. However, error correction using the method described in U.S. Pat. No. 5,949,831 has limits such that the method finds an error type depending only on the error event filter.
According to a technology disclosed in the paper entitled, “Noise predictive maximum likelihood detection combined with parity-based post-processing,” IEEE Trans. Magnetics, Vol. 37, No. 2, March 2001, a dominant error event may be corrected by including a parity-based post-processor.
According to a technology disclosed in U.S. Pat. No. 6,427,220, a post-processor based on cyclic redundancy checking (CRC) may accurately detect positions of errors using CRC codes.
According to a technology disclosed in U.S. Pat. No. 6,931,585, an accurate value of a maximum likelihood distance penalty (MLDP) is obtained using an accumulation of branch metric (BM) values. However, the technology disclosed in U.S. Pat. No. 6,931,585 may require a large memory capacity for storing parameters as media density is increased.
However, a conventional method of signal processing cannot adaptively decrease signal-dependent noise according to media characteristics.